Online, a couple websites came up in a search for the AOE DE Manual:
https://www.google.com/search?q="Age+of+empire+definitive+edition+user+manual"&rlz=1C1CHBF_enUS930US930&oq="Age+of+empire+definitive+edition+user+manual"&aqs=chrome..69i57j0i22i30.1316j0j9&sourceid=chrome&ie=UTF-8
They look like junk websites, but the first one quotes a link from the “Assyria” entry on the AOE Wiki page, and I don’t know if that quote is in a real AOE DE Manual or not.
I am not aware of any real AOE DE Manual, but there are old AOE1 ROR (1999) manual copies online.